ABSTRACT:
In order to increase the utilisation of Irish timber in construction and novel engineered wood products, the mechanical and physical properties of the material must be established. For timber products used for structural applications, the fundamental properties are the modulus of elasticity, bending strength, density and dimensional stability, as these define the structural grade of the material. In order to develop engineering design models for applications such as reinforced timber, knowledge of the nonlinear stress-strain behaviour in compression is also required.
The paper presents the programme and results of an ongoing research project ‘Innovation in Irish Timber Usage’ which focuses on the characterisation of Sitka spruce as it is the most widely grown species in Ireland. In the past, a number of studies have been conducted to determine the properties of Irish-grown Sitka spruce. Nevertheless, due to the changes that have taken place in silvicultural practices since the publication of these studies, there is a need to determine how these properties have changed. This paper presents the data gathered from historical studies together with the results of an extensive test programme undertaken to characterise the properties of the present resource.
Moreover, the study preliminary examines the potential use of Irish grown Sitka spruce in novel timber products. Construction applications, such as fibre-reinforced polymer reinforced timber elements and connections, and cross-laminated timber are investigated.
KEY WORDS: Sitka spruce, timber properties, reinforced timber, Cross-laminated timber
1 Introduction
Due to the increaseding focus on the use of sustainable construction materials to meet environmental targets related to efficient energy use and emissions, a significant opportunity exists for the Irish wood products sector. In 2012, the Irish forestry and forest products sector generated €2.2 billion in annual output, representing 1.3% GDP, and employed approximately 12,000 people [1]. Moreover, there exists a substantial potential to expand production. According to COFORD [2] half of the forest estate is less than 25 years old and further expansion of forest cover is planned by policymakers. Forest products to a value of €303 million were exported: including €73 million worth of sawn softwood and €179 million worth of wood-based panels. In general, 89% of the wood-based panels were exported [1]. The supply of roundwood from Irish forests is projected to increase from 3903 million m3 in 2011 to 7110 million m3 in 2028. These figures show the potential of Irish forests to provide increased and sustainable supplies of wood products [3]. Increased sales of existing products and the development of new markets at home and abroad for new added-value wood products will lead to job creation across the sector.
In order to increase the utilisation of Irish timber in construction, the mechanical and physical properties of the material must be established. For timber products used for structural applications, the fundamental properties are the bending modulus of elasticity (MOE), the modulus of rupture (MOR), the density and the dimensional stability as these define the structural grade of the material. In order to develop engineering design models for reinforced timber, knowledge of the nonlinear stress-strain behaviour in compression is also required.
As part of the project ‘Innovation in Irish Timber Usage’, funded by the Department of Agriculture, Food and Marine of the Republic of Ireland under FIRM/RSF/COFORD scheme, all of the available historical data on the properties of Irish Sitka spruce, from published and unpublished sources, is being collated. Moreover, testing of a large number of samples is being carried out to establish the mechanical and physical properties of the current resource. Furthermore, in order to investigate the potential for new add-value timber construction products, the two key research areas are addressed, namely, Fibre-Reinforced Polymer (FRP) reinforced timber and Cross-Laminated Timber (CLT).
2 CHARACTERIsATION of irish timber
2.1 Introduction
The focus of this paper will be on timber from Sitka spruce as it is the most widely grown species in Ireland. Irish-grown Sitka spruce is characterised as a fast growing, low density species due to the rapid growth condition in Ireland and short rotation length. As a result of these growth conditions, the most common structural grade achieved by Irish-grown Sitka spruce is C16 grade.
In the past, a number of studies have been conducted on the properties of Irish-grown Sitka spruce [4-16]. This species is native to a narrow belt of the Pacific North West coast of North America, along Alaska in the north, down through British Columbia, Washington and Oregon to California. Due to similarities in climate between this region and Ireland, it was first introduced to Ireland in 1831. The wide ranging site types suited to growing Sitka spruce vary from very fertile mineral to impoverished peaty soils [17].
A large study (the SIRT project) on Scottish Sitka spruce was undertaken in Scotland in recent years [18]. This has resulted in the publication of a report by the Forestry Commission entitled ‘Wood properties and uses of Sitka spruce in Britain’ [19]. This report is important for Ireland as the growing conditions in Scotland are similar to those in Ireland and the likelihood is that the physical and mechanical properties of the timber produced in both countries will be comparable.
In the following sections, properties of Irish Sitka spruce from selected studies are presented.
2.2 Mechanical properties
Investigations and grading results carried out by sawmills confirm that Irish Sitka spruce meets predominantly the requirements of strength class of C16. Table 1 presents the main strength requirements for C16 according to EN 338 [20]
Table 1. EN 338 [20] main characteristic values for C16.
Bending strength (fm,k) / Compression strength parallel (fc,0,k) / Mean modulus of elasticity parallel (E0,mean) / Mean density (ρmean)16 N/mm2 / 17 N/mm2 / 8 kN/mm2 / 370 kg/m3
Picardo [4] undertook a large-scale testing programme to evaluate the influence of a number of classifying variables such as yield class (mean of cubic metres of solid stem wood added to an area of woodland per hectare per year [m3/ha/yr]), section size and forest on the strength and stiffness of Irish Sitka spruce. From tests on 1487 planks, he found that only section size had a practical influence on the MOR.
Ní Dhubháin et al. [5] examined the influence of compression wood on the bending MOE and MOR properties. This study had a relatively small sample size of 100 specimens of single cross-sectional size. They found that increased percentages of compression wood resulted in a decrease in MOE but appeared to have no effect on MOR.
Picardo [6] conducted another large study involving the machine grading of about 5000 pieces of timber into different strength classes. This study found that the yields for C14 and C16 were very high and the thickness has a significant impact on the yield. He did not undertake destructive mechanical tests.
Lucey et al. [7] investigated the utilisation of Irish grown Sitka Spruce timber (from forests in County Galway) in I-joists. It was reported that the stiffness of the I-joist has a high correlation to the stiffness of both tension and compression flanges. There was only a low level correlation between the strength of the web material and the strength of I-joist. Therefore, extensive testing programme was carried out on specimens used for the flanges. The MOR of the flange materials, presented in Table 2, was measured as described in EN 408 [21]. The average MOR results ranged from 22.2 N/mm2 to 25.4 N/mm2 for all cross-sectional sizes. However, the dimensions had an influence on the standard deviation of and 5-percentile of the MOR, which was lower for the smaller cross-sections.
Table 2. MOR results for square beams [7].
Specimen type & size / No. of specimens / Aver. MOE [N/mm2] / S. D. [N/mm2] / 5-percen. [N/mm2]compression flanges:
44 x 44 mm2 / 31 / 22.2 / 4.6 / 13.5
tension flanges:
44 x 44 mm2 / 31 / 22.3 / 3.9 / 15.5
compression flanges:
65 x 65 mm2 / 11 / 24.8 / 2.0 / 21.9
tension flanges:
65 x 65 mm2 / 11 / 24.4 / 4.0 / 18.1
compression flanges:
65 x 65 mm2 / 10 / 25.5 / 2.1 / 22.9
tension flanges:
65 x 65 mm2 / 10 / 25.4 / 2.8 / 20.9
The MOE parallel to grain was also measured in specimens stressed in both tension and compression. The results differed depending on cross-section size, and reached almost 8 MPa for the bigger sizes and 7 MPa for the smaller ones. These results for MOE testing are shown in Table 3.
Table 3. MOE results for square beams [7].
Specimen type & size / No. of specimens / Aver. MOE [N/mm2] / S. D. [N/mm2] / 5-percen. [N/mm2]compression flanges:
44 x 44 mm2 / 29 / 7027 / 1026 / 5148
tension flanges:
44 x 44 mm2 / 29 / 7106 / 945 / 5610
compression flanges:
65 x 65 mm2 / 11 / 7533 / 627 / 6770
tension flanges:
65 x 65 mm2 / 11 / 7531 / 1319 / 5573
compression flanges:
65 x 65 mm2 / 10 / 7997 / 606 / 7180
tension flanges:
65 x 65 mm2 / 10 / 7999 / 885 / 6696
In addition to bending properties, Lucey et al. [7] examined compression parallel to grain and tension strengths for various sizes of samples using Irish Sitka spruce. The results of these mechanical properties are summarised in: Table 4 – compression strength parallel to grain and Table 5 – Tension strength.
Table 4. Compression strength parallel to grain results [7].
Specimen size [mm x mm x mm] / No. of specimens / Aver. MOE [N/mm2] / S. D. [N/mm2] / 5-percen. [N/mm2]44 x 19 x 19 / 31 / 24.9 / 4.8 / 17.2
65 x 40 x 39 / 11 / 24.4 / 1.8 / 22.4
Table 5. Tension strength results [7].
Specimen size [mm x mm x mm] / No. of specimens / Aver. MOE [N/mm2] / S. D. [N/mm2] / 5-percen. [N/mm2]44 x 19 x 10 / 31 / 23.9 / 4.3 / 16.3
65 x 40 x 20 / 11 / 24.6 / 2.9 / 20.2
65 x 40 x 20 / 10 / 25.1 / 2.1 / 22.2
Raftery and Harte [8] undertook a comprehensive study in order to assess the relationship between mechanical properties and physical characteristics in the longitudinal direction on clear and in-grade samples. Parameters, which were studied, included density, knot area ratio, MOE and ultimate strength. The results of this investigation might be essential in terms on the future development, design and optimisation of engineered wood products from Irish Sitka Spruce. It was concluded that MOE was the most highly correlated parameter to the tensile strength for both clear and in-grade specimens. Furthermore, the knot area ratio had a considerable influence on the strength of the timber both in compression and tension. Density was more highly correlated to ultimate compressive strength than ultimate tensile strength in clear wood specimens. In addition, the authors reported that MOE in tension had a poor correlation to the density of clear wood and was also poorly correlated to the knot area ratio and the density of in-grade specimens.
Moreover, testing of Irish timber has been undertaken in Irish third level institutions as part of the research for MSc and PhD theses. Not all of this data has been published in the available literature. Nevertheless, some of these results are shown in the following paragraphs.
The quality of Irish Sitka spruce was extensively investigated by Evertsen [9] using destructive and non-destructive methods. In order to determine the MOE and MOR, static four point bending tests were carried out on over 200 planed planks of different sizes taken from woodlands of yield classes 16 and 20. The moisture content of the samples during test was generally at 15 ± 2%. The mean, maximum, and minimum MOE and MOE results, including standard deviation, for different yield classes are presented in Tables 6 and 7.
Table 6. MOE from destructive tests [9].
Yield class / No. of specimens / Mean MOE [N/mm2] / S. D. [N/mm2] / Min. [N/mm2] / Max. [N/mm2]16 / 64 / 8623 / 2757 / 1044 / 18424
20 / 116 / 9291 / 2511 / 3707 / 15772
16&20 / 180 / 9053 / 2601 / 1044 / 15772
Table 7. MOR from destructive tests [9].
Yield class / No. of specimens / Mean MOR [N/mm2] / S. D. [N/mm2] / Min. [N/mm2] / Max. [N/mm2]16 / 64 / 24.4 / 9.4 / 0.4 / 44.5
20 / 116 / 26.3 / 7.5 / 7.2 / 47.8
16&20 / 180 / 25.6 / 8.2 / 0.4 / 47.8
For the non-destructive determination of the mechanical strength properties, an ultrasonic testing method was used by Evertsen [9]. This technique was developed by Bucur [10, 11], who had found that the MOE for small clear specimens had a high correlation with the ultrasound propagation speed in increment core sized samples. 300 small clear specimens were tested including 240 of yield class 16 and 60 of yield class 20. The obtained average MOE values were 8639 N/mm2 and 7677 N/mm2 for the first and second batch, respectively, and the average MOR values were 73.8 N/mm2 and 67.2 N/mm2, respectively.
Subsequently, tests results published by Patrick [12] confirmed that Sitka spruce has a brittle mode of failure in axial tension influenced by larger knot occurrence. The characteristic in-grade strength of 14.4 N/mm2 was obtained in comparison to mean in-grade strength of 27.1 N/mm2 that was approximately half of the strength for clear timber. On the other hand, the mean and characteristic compressive strengths showed less variability reaching 30.6 N/mm2 and 21.0 N/mm2, respectively, for in-grade samples. The value of MOE in compression of 9125 N/mm2 was found.